What Is an OTDR Launch Cable Box?
An OTDR launch cable box (also called a dead zone eliminator, launch fiber reel, or OTDR mandrel) is a reel of optical fiber – typically 100m, 300m, 500m, 1000m, or 2000m in length – that is connected between the OTDR and the fiber under test. It is one of the most important accessories for accurate OTDR fiber testing.
Without a launch cable box, the OTDR cannot accurately measure the first connector and the first few hundred meters of the fiber link due to the dead zone – a blind spot created by the strong Fresnel reflection from the OTDR’s output connector.
Understanding the OTDR Dead Zone Problem
When an OTDR launches a light pulse into a fiber, the connector at the OTDR’s output port creates a very strong Fresnel reflection that temporarily saturates the OTDR’s receiver. During this saturation period – which can last for several hundred meters – the OTDR is effectively blind and cannot detect any events (splices, connectors, bends, or breaks) in the fiber.
This blind region is called the dead zone, and it has two components:
- Event Dead Zone (EDZ) – The minimum distance after a reflective event where the OTDR can detect the next event. Typically 1–5 meters for modern OTDRs.
- Attenuation Dead Zone (ADZ) – The minimum distance after a reflective event where the OTDR can accurately measure the loss of the next event. Typically 5–25 meters for modern OTDRs.
For FTTH testing, where the first connector or splice may be only a few meters from the OTDR, the dead zone makes it impossible to accurately characterize the near-end of the fiber link without a launch cable box.
How a Launch Cable Box Solves the Dead Zone Problem
By inserting a known length of fiber between the OTDR and the fiber under test, the launch cable box pushes the dead zone beyond the first connector of the fiber link. The OTDR can now accurately measure all events from the very beginning of the fiber link, including the first connector and the first splice.
The launch cable also provides a reference backscatter level that allows the OTDR to accurately measure the loss of the first connector on the fiber under test.
How to Choose the Right Launch Cable Length
The required launch cable length depends on the OTDR’s dead zone specification and the pulse width being used:
- 100–300m – Suitable for low-power OTDRs with short dead zones. Minimum recommended for FTTH testing.
- 500m – The most popular choice for FTTH and short-haul fiber testing. Sufficient for most modern OTDRs at standard pulse widths.
- 1000m – Recommended for high-power OTDRs or when using long pulse widths for extended range testing.
- 2000m – For very high-power OTDRs or long-haul telecom testing with maximum pulse widths.
Rule of thumb: The launch cable should be at least 3–5 times longer than the OTDR’s attenuation dead zone specification.
Launch Cable Connector Types
Launch cable boxes are available with different connector types to match your OTDR port and the fiber under test:
- SC/UPC to SC/UPC – Most common for FTTH testing
- SC/APC to SC/APC – For APC connector networks
- SC/UPC to SC/APC – For mixed connector networks
- FC/UPC to SC/UPC – For OTDRs with FC ports
Recommended OTDR Launch Cable Boxes
Browse our range of OTDR launch cable boxes available in 500m, 1000m, and 2000m lengths with SC/UPC, SC/APC, and FC connector options. All launch cables use high-quality single-mode fiber (G.652D) for accurate, low-loss OTDR testing.
How to Use an OTDR Launch Cable Box: Step-by-Step
- Clean the connectors – Clean both the OTDR port and the launch cable connectors using a one-click cleaner before connecting.
- Connect the launch cable to the OTDR – Connect one end of the launch cable box to the OTDR’s fiber port.
- Connect the launch cable to the fiber under test – Connect the other end of the launch cable to the first connector of the fiber link being tested.
- Set the OTDR parameters – Set the distance range to include the full length of the launch cable plus the fiber under test.
- Run the OTDR measurement – The OTDR trace will show the launch cable as a flat backscatter region, followed by the first connector of the fiber link and then the fiber under test.
- Set the start marker – Place the OTDR’s start marker at the connector between the launch cable and the fiber under test. All measurements will be referenced from this point.
Frequently Asked Questions
Do I need a launch cable box for all OTDR testing?
You need a launch cable box whenever you need to accurately measure events near the beginning of the fiber link – which is virtually always the case for FTTH and telecom fiber testing. The only exception is when testing very long fiber links where the near-end dead zone is not a concern.
Can I use any fiber as a launch cable?
Technically yes, but a purpose-built launch cable box is much more convenient and reliable. It uses high-quality, low-loss fiber wound on a compact reel with clean, properly terminated connectors.
Do I need a receive cable box as well?
For bidirectional OTDR testing (testing from both ends), you also need a receive cable box at the far end of the fiber link. This eliminates the far-end dead zone and allows accurate measurement of the last connector on the fiber link.
Conclusion
An OTDR launch cable box is an essential accessory for any fiber optic technician who uses an OTDR. Without one, you cannot accurately characterize the near-end of a fiber link – which is where many installation problems occur. Investing in a quality launch cable box will ensure your OTDR measurements are accurate, complete, and reliable.
Browse our full range of OTDR launch cable boxes at splicingmachine.net – worldwide shipping available.
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